1. Field of the Invention
This invention relates to a method and structure for interconnecting two integrated optical waveguides that lie in different vertical planes within a multilayer optical circuit. More specifically, this invention relates to an optical via that transfers optical power from one waveguide to a vertically adjacent waveguide, the power transfer being fabrication tolerant, polarization tolerant, wavelength tolerant, and dimensionally tolerant.
2. Description of Related Art
Planar optical circuits are comprised of optical waveguides and devices that confine light to propagate primarily along a two-dimensional plane. Light is confined by an optical waveguide. The waveguide is comprised of a core having a certain refractive index, and a cladding surrounding the core on all sides. The cladding can be different on the top, sides, and bottom of the core, but at all locations it will have a refractive index that is lower than the refractive index of the core. The plane of propagation is parallel to the substrate on which the dielectric layers comprising the waveguides are fabricated. It is possible to have numerous planar layers each having waveguides vertically positioned or stacked one on top of the other. This increases integration densities. These stacked layers are often called multilayer optical circuits, or vertically integrated circuits.
Vertical integration is often used in silicon electronics both for device layers and for electrical interconnect (wiring) layers. Electrical signals can be routed from one layer to another layer by the use of electrical vias, which are electrical connections in the vertical direction. For an electrical via to work properly, it is sufficient to have low resistance electrical connectivity between the layers through the optical via.
Optical vias, which transfer optical signals between adjacent planes, are not straight forward. Unlike electrical vias, it is not sufficient to have a continuity of optical core material for the photons to follow because photons do not behave like electrons which simply follow a path of least resistance. Photons can not traverse right angle turns without significant scattering loss.
A planar optical waveguide circuit comprised of two guiding layers is depicted in FIGS. 1A to 1C. FIGS. 1A to 1C show three different cross sections of a portion of the optical waveguide circuit. FIG. 1A shows a top down schematic view pointing out the two waveguides (110, 111) that are on different planar layers. FIG. 1B shows a lateral cross section of the structure, which in general comprises two waveguide cores (110, 111), a substrate (116), a lower cladding (115), a buffer layer between the two planar light guiding cores (113), a top cladding (112) and cladding around the two light guiding cores (114, 112). FIG. 1C shows a longitudinal cross section schematic of a portion of a two-layer optical circuit. Circuits similar to FIGS. 1A to 1C, having one or multiple layers, are called “planar circuits” because propagation takes place mainly in a plane. The various waveguides in different vertical planar layers do not interact except when they are close enough such that the optical modes supported by the waveguides can interact with each other. This interaction range is usually limited to a distance smaller than several optical wavelengths. The interaction is often called “evanescent” interaction because the evanescent fields of the modes supported by the waveguides interact. By extension, multilayer circuits are similar to those in FIG. 1, having multiple guiding layers surrounded by cladding layers, and adjacent guiding layers separated by buffer layers.
An optical via is a structure that allows passage of an optical signal from one plane to another with low loss. One method to accomplish this is the vertical directional coupler shown in FIGS. 2A to 2C. FIGS. 2A to 2C are similar to FIGS. 1A to 1C, in that the structure described has two vertical waveguiding planar layers that are separated by a buffer layer. FIGS. 2A to 2C describes a specific two layer optical circuit called a vertically coupled directional coupler. In this case two waveguides in two planes are parallel to each other and directly above one another. The waveguides co-propagate together over some length. They are close enough so that the optical fields or modes in the two waveguides can interact evanescently. If the waveguides have identical propagation constants, also called synchronous, they will exchange full power over a certain length called the beat length. The beat length is a function of various geometrical waveguide parameters such as refractive index, geometric dimensions, and buffer layer thickness. The beat length is also a function of wavelength and polarization. If the length of interaction is longer than a beat length, power that has been coupled from one waveguide to another will couple back into the first waveguide, which is undesirable. If the length of interaction is shorter than a beat length, full power transfer will not be achieved. It is therefore essential to correctly design the directional coupler to be exactly one beat length long at the wavelength of interest. The directional coupler type of via is simple and short. Its drawbacks are that it is wavelength and polarization sensitive and it is sensitive to all fabrication imperfections that cause the two waveguides to not be identical, such as, for example, changes in refractive index, dimensions, or buffer layer thickness.
U.S. Pat. No. 3,785,717 to M. Croset et al. describes a multilayer optical circuit comprised of directional couplers, similar to FIGS. 2A to 2C. The directional couplers are used to transfer optical power between layers. The difficulty of directional couplers is that they are very fabrication sensitive and also naturally wavelength dependent or narrow band, and polarization dependent. The method specifically described in U.S. Pat. No. 3,785,717 is also only limited to diffused-type waveguides, which are no longer used in state of the art optical circuits.
U.S. Pat. No. 4,472,020 to V. L. Evanchuk et al. describes a method for making monolithic circuits having waveguides on multiple layers. The method mainly pertains to fabrication methods to realize multilayer structures. Via-like structures are discussed for the transfer of optical power between layers, and these structures amount to corner reflectors or mirrors. In practice, integrated optic corner reflectors and mirrors are very difficult to fabricate and are inherently very lossy. This loss can not be overcome easily. Further, corner reflectors and mirrors need very high index contrast materials, or metallic layers which are inherently absorptive.
U.S. Pat. No. 6,236,786 to H. Aoki et al. describes a dual layer optical circuit where the two layers are connected by a through hole. As in U.S. Pat. No. 4,472,020 discussed above, corner reflectors or mirrors are used to transfer power between the two layers, using the through hole as a vertical light pipe to confine the light as it traverses from one layer to the other. In practice, these structures are very lossy, especially for single mode waveguides, and required fabrication methods that are unconventional, and, therefore, not suitable for mass production.
U.S. Pat. No. 3,663,194 to B. Greenstein et al. describes multilayer optical circuits. Although the invention teaches methods to realize multilayer circuits, these multilayers do not communicate with one another. Rather they are independent.
U.S. Pat. No. 4,070,516 to H. D. Kaiser describes a multilayer optical circuit and module. As in U.S. Pat. No. 4,472,020 discussed above, corner reflectors, mirrors, and right-angle bends are described for use in coupling the multilayers and changing the direction of the light signal in each layer. To date, such mirrors and corner reflectors have not shown promise in practice, and, therefore, other means must be invented. Further, light can not fundamentally be guided around a right-angle bend without loss and scattering. Photons, unlike electrons, can not be induced to follow right angle bends.
U.S. Pat. No. 6,650,817 to V. Murali describes a multi-level waveguide. The waveguides on multi-levels are interconnected by etching holes from one layer to another and these holes filled with optically transparent and optically guiding material. This method uses unconventional fabrication methods and is unsuitable for volume manufacturing. Further, as in earlier cited prior art above, light is forced to be guided around right-angle bends, which fundamentally are not low loss.
None of the prior art provides for efficient low loss power transfer between optical waveguides in a multilayer optical circuit. This invention discloses, for the first time, an efficient optical via that provides for low loss, fabrication tolerant, and broad wavelength usage power transfer structure to connect optical waveguides on different layers of a multilayer optical circuit.